1. Field of the Invention
The invention relates to an arrangement in a vessel for loading or unloading of a flowable medium, especially oil, the vessel being provided with a submerged downwardly open receiving space for receiving and securing a buoy which is anchored to the sea bed and is coupled to at least one transfer line for medium.
2. Background Information
There are previously known various embodiments of loading/unloading systems for the transfer of oil by means of a submerged buoyancy unit or buoy which, during operation, is received and secured in a submerged receiving space on a vessel, especially at the underside thereof. A system comprising a vessel of the above-mentioned type is known from e.g. U.S. Pat. No. 4 604 961 (corresponds to Norwegian patent No. 167 906). This known system is based on a vessel having a through-going deck opening in a central region of the vessel, the lower part of the through opening forming the receiving space for a mooring element in the form of a submerged buoy. In the receiving space there is arranged a rotating body (turret) which is rotatably mounted in the hull of the vessel and is designed for receipt and attachment of the mooring element, the latter to this end being provided with a hydraulically actuated locking mechanism for attachment to the rotating body. Further, the vessel is provided with a derrick for the lowering of a retrieval string having a retrieval connector at its lower end for interconnection with the mooring element, so that this may be pulled up and into the receiving space. The interconnection is obtained in that the mooring element is provided with a conical centering receptacle having a socket arranged at the bottom wherein the retrieval connector may be received and secured, e.g. by means of a bayonet lock. The lower end of the retrieval string preferably is provided with sonar and TV equipment to ensure positioning of the retrieval connector in the centering receptacle.
The known system is encumbered with some drawbacks which will be discussed below.
As mentioned, the vessel of the known system is based on a through-going deck opening, which reduces the strength of the vessel and poses demands for additional reinforcements in the bottom and the deck of the vessel. Experience has also shown that ships having a through-going deck opening are subject to fatigue in the hull. A ship having such a through-going shaft or opening has to be constructed with its definite objective kept in view, and it will be a very expensive solution to carry out modifications of already existing ships to provide them with such an opening.
Since the rotating body is attached to the vessel under water, this requires divers for inspection and minor maintenance. Major maintenance requires docking of the vessel. Because of the fact that the rotating body is mounted to the vessel, there arise large frictional forces which are to be overcome by torques from the mooring element. These torques are relatively large due to the large outer diameter of the rotating body, and this results in correspondingly large loads. Further, it may result in uncontrolled rotation of the system because of large inertial forces, so that it becomes necessary to use a braking system for retaining the rotating body. In case of desired rotation the braking system is then released, and the rotating body is rotated in a controlled manner by means of active drive.
Further, the known system has a small ability to absorb moments caused by the horizontal mooring forces, something which results in a substantial risk for jamming actions in the mounting arrangement.
The hydraulically actuated locking mechanism which is arranged on the mooring element requires divers for connection of the control hydraulics. Diver operations in connection with connection and disconnection render the use of the system as a transport system impossible, when using shuttle tankers. Further, there is a big risk for faulty operation and damages in case of uncontrolled disconnection. In case of breakage of the hydraulic system there is no possibility for the connection of a back-up or auxiliary device.
It is an object of the invention to provide an arrangement in a vessel of the introductorily stated type making it possible to carry out connection and disconnection between vessel and buoy in a quick and simple manner, even in bad weather.
Another object of the invention is to provide an arrangement making possible a very quick disconnection of the buoy if a weather limitation should be exceeded, so that the utilized vessels can be operated as usual ships with respect to service, repair and classification.
A further object is to provide an arrangement giving low total investment and simultaneously the possibility to undertake repairs and replacement of parts on board the vessel, without disconnection of the buoy.
A further object is to provide an arrangement making possible a relatively simple and reasonable rebuilding of existing vessels for adaptation to the utilized buoy loading system.
A still further object of the invention is to provide an arrangement giving a high security in operation and a low risk for contaminating spill.